ImportError: undefined symbol when importing swigged c++-class in python

Question

I try to use a c++-class for socket communication in Python. Therefore, I created a class that uses nngpp . When importing the swigged file to python, I get the ImportError: undefined symbol: nng_msg_insert. The definition of the class is:

/* commclass.h */
#include <nngpp/nngpp.h>
#include <nngpp/protocol/req0.h>
#include <nngpp/protocol/rep0.h>
#include <nngpp/msg_body.h>
#include <nngpp/msg_header.h>
#include <nngpp/msg.h>
#include <nngpp/socket.h>
#include <nngpp/view.h>
#include <string>
#include <nlohmann/json.hpp>
//#include <thread>
#include <iostream>
#include <cstdio>
#include "/usr/local/include/nng/nng.h"
//#include <memory>
#include <chrono>
using json = nlohmann::json;

class CommIF
{
private:
    nng::socket socket;
    nng::msg message;
    int msg_size;
public:
    CommIF(const std::string option, std::string ipToListen, std::string ipToDial)
    {
        message = nng::make_msg(0);
        if (option.compare("rep") == 0)
        {
            socket = std::move(nng::rep::v0::open());
        }
        else if (option.compare("req") == 0)
        {
            socket = std::move(nng::req::v0::open());
        }
        else
        {
            printf("EXCEPTION");
        }
        socket.listen(ipToListen.c_str());
        bool connected = false;
        while (connected == false)
        {
            try
            {
                socket.dial(ipToDial.c_str());
                connected = true;
                std::cout << "successfully connected\n";
            }
            catch (const nng::exception &e)
            {
                std::cerr << e.what() << "; retry in 1 s" << '\n';
                //std::this_thread::sleep_for(std::chrono::seconds(1));
            }
        }
        msg_size = 0;
    }
};

The interface file for swig is:

/* commclass.i */
%module commclass
%{
#include "src/commclass.h"
%}
%include "src/commclass.h"

I then start with the command python3 build_commclass.py build_ext --inplace the build process. The file build_commclass.py is the following

from distutils.core import setup, Extension
import os

name = "commclass"
version = "0.0.1"               
os.environ["CC"] = "g++"

setup(name = name, version = version, ext_modules = [Extension(
name = '_commclass', 
sources = ["commclass.i"],#"src/commclass.h"],
include_dirs = ['src'],#'/home/user1/Documents/extLibs','/usr/local/include'],
swig_opts = ["-c++", "-modern"]
)])

When I now import the the class to python, I get the error mentioned above. I searched a lot on google and stackoverflow and I am quite sure this is a linker issue. I also tried a lot of different things with the compiler and linker options of distutils.core, but I didn't find a solution.

Edit 1: I now changed the interface file as following

/* commclass.i */
/* module*/
%module commclass
%{
#include "/usr/local/include/nng/nng.h"
#include "src/nngpp/nngpp.h"
#include "src/nngpp/protocol/req0.h"
#include "src/nngpp/protocol/rep0.h"
#include "src/nngpp/socket.h"
#include "src/nngpp/msg.h"
#include "src/nngpp/aio.h"
#include "src/nngpp/aio_view.h"
#include "src/nngpp/msg_body.h"
#include "src/nngpp/msg_header.h"
#include "src/commclass.h"
%}
%include "/usr/local/include/nng/nng.h"
%include "src/commclass.h"

I now get the same error when importing in python, but the undefined symbol changed. It is now nng_aio_set_iov . Both nng_msg_insert and nng_aio_set_iov are defined in the file nng.h which I have now included. I am confused now.

Answer 1

SWIG only generates interfaces for definitions that are directly specified by %include by default. nng_msg_insert is not defined in src/commclass.h . You need additional %include statements to bring in the definition.

Note you can change the default to recurse into all #include statements with the -includeall SWIG flag, but you generally don't want the entire <iostream> , <cstdio> , <string> , etc. interfaces to be wrapped and it likely won't work without a lot of additional effort.